The goal of this multidisciplinary research program is to examine factors that contribute to the learnability of the sound system for speech disordered children. Both child-specific factors (properties central to a given child) and language-general factors (properties common to all languages) will be considered. These factors will be examined specifically within the framework of single-subject methodology and will be evaluated through descriptions and experimental clinical treatment of disordered sound systems. Four projects are planned. Projects I and II will examine the child-specific factor of productive phonological knowledge. Project I will test the hypothesis that productive knowledge takes precedence over treatment variables in facilitating sound learning. Project II will test the hypothesis that direct clinical treatment is not required in cases where a child displays productive knowledge of sounds in the form of systematic, although nonperceptible, acoustic phonetic distinctions. Project III will examine a second child-specific factor, perceptual, phonological knowledge. Two hypotheses will be tested: (a) children perceive distinctions in their own production of sounds that are not perceptible to other listeners and (b) treatment of perceptual knowledge contributes equally to the learnability of sounds as treatment of productive knowledge. Project IV will test the hypothesis that language universals reliably predict sound learning. Project IV will also examine the potential relationship between child-specific and language-general factors in sound learning. Clinically, the results of these four projects will help identify factors that may be used in treatment to enhance sound learning for speech disordered children and other language-learning populations. Theoretically, these results will potentially isolate the necessary and specific properties of sound systems and will identify processes essential to learning a sound system.